Roads used to be made from clay, earth or chalk, but these materials were very messy. John McAdam thought ... substances to examine such as clay, mud, soil, tarmac, rubber, pebbles and so on.

Roads used to be made from clay, earth or chalk, but these materials were very messy. John McAdam thought ... substances to examine such as clay, mud, soil, tarmac, rubber, pebbles and so on.